Altitude is defined as a vertical distance above a reference point. Barometric altitude, central to fields like aviation and meteorology, measures this height using atmospheric pressure. This method relies on the physical property that air pressure naturally decreases the higher you ascend. Measuring this pressure change provides a reliable, indirect method for determining vertical position by converting atmospheric force into a height value.
Defining Barometric Altitude
Barometric altitude is the height above a fixed reference level calculated exclusively from a measured atmospheric pressure reading. This operates on the principle that the column of air above a point weighs less as altitude increases, causing a drop in pressure. A pressure altimeter, essentially a calibrated barometer, converts this lower pressure reading into a corresponding higher altitude in feet or meters. This calculated value is referenced against a known pressure at sea level, forming a baseline. Barometric altitude is distinct from True Altitude (the actual physical height above mean sea level, MSL). Since the reading is derived from a pressure model, it represents a calculated height rather than a direct physical measurement.
Factors Influencing Barometric Readings
The atmosphere is a dynamic system, meaning pressure changes are not solely due to changes in physical height, which introduces variability into barometric readings. Two primary influences that alter the measurement are weather systems and temperature. High and low-pressure zones drastically affect local pressure, causing an altimeter to indicate a height change even if the device is stationary. An approaching low-pressure system makes an altimeter read higher than the true altitude, summarized by the adage, “High to low, look out below.” Warm air is less dense than cold air, affecting the rate at which pressure changes with altitude. Since altimeters are calibrated to a standard temperature, deviations require correction. To achieve an accurate reading relative to the ground or sea level, the altimeter must be calibrated by inputting the current local sea-level pressure, often referred to as QNH.
Practical Applications of Barometric Measurement
Barometric altitude measurement provides a reliable means of vertical positioning across various fields. In aviation, barometric altimeters are the primary method for maintaining safe vertical separation between aircraft. Pilots continuously adjust their altimeter settings to reflect local atmospheric conditions, ensuring all aircraft reference the same pressure datum. Weather forecasting relies heavily on barometric pressure changes, as these shifts are indicators of approaching weather fronts and systems. A rapid drop in pressure often signals a storm, making the measurement foundational for short-term prediction. Recreational users, such as hikers and mountaineers, utilize hand-held altimeters to track their progress. These devices require periodic calibration when reaching a known elevation to maintain accuracy against changing weather conditions.
Specialized Uses in Performance
Beyond simple height measurement, barometric pressure is used to calculate specialized performance metrics in aviation.
Pressure Altitude
Pressure Altitude is the altitude indicated when the altimeter is set to the standard atmospheric pressure of 29.92 inches of mercury (inHg) or 1013 hectopascals (hPa). This standard datum serves as a common reference point for all aircraft operating above a certain altitude, typically 18,000 feet, where pilots set their altimeters to 29.92 inHg for uniformity.
Density Altitude
Pressure Altitude is the necessary first step for calculating Density Altitude, a more practical metric for performance assessment. Density Altitude is defined as the Pressure Altitude corrected for non-standard temperature and humidity, representing the altitude at which the air density matches the International Standard Atmosphere. Hotter temperatures or higher humidity cause the air to become less dense, which is equivalent to operating at a higher altitude. For example, an airport at sea level on a hot day may have a Density Altitude of 5,000 feet, meaning an aircraft performs as if it were taking off from that higher elevation. High Density Altitude results in reduced engine power, less lift from the wings, and a longer takeoff roll, making it a consideration for flight planning.